1. Field of the Invention
The present invention relares to an image reader which employs a plurality of image pickup elements such as CCD line sensors thereby to read image data along each scanning line.
2. Description of the Prior Art
An image reader used in the field of, e.g., process printing is required to be remakably excellent in image reproducibility, and hence it is necessary to read images through image pickup element which is assembled by extremely large numbers of unit elements. However, only a limited number of such unit elements can be assembled in a single image pickup element in the present state of art. Therefore, such a system as hereafter descried is employed in these fields.
In this system, as shown in FIG. 1, a read range R of an original 1 in the main scanning direction is split into a plurality of sections 2a and 2b, so that image data in the respective sections 2a and 2b are read by a plurality of line sensors 3a and 3b. Then the data are so connected as to obtain a reproduced image. Shch a system is hereinafter referred to as "split read system". In this system, it is substantially impossible to perform mutual arrangement of the line sensors 3a and 3b in the units of pixels (e.g., 7 .mu..times.7.mu.), and hence portions shown by .DELTA.R in FIG. 1 are overlappingly read by the two line sensors 3a and 3b. As shown in FIG. 2, the line sensors 3a and 3b are switched at the positions of specific pixels (i-th pixels in the case of FIG. 2) corresponding to the overlapping portions .DELTA.R, thereby to connect the image data.
However, it is difficult to perform relative arrangement of the line sensors 3a and 3b as hereinabove described and the relative positions thereof are secularly changed, whereby the i-th pixel overlaps with an (i+1)-th pixel by, e.g., 1/2 pixel as shown in FIG. 3. Thus, data for a pixel included in the original may appear in two pixels in the reproduced image. FIG. 4 is an enlarged view of images for illustrating such states. When read pixels overlap with each other in connection of image data for an original as shown at (a) in FIG. 4, the reproduced image is irregulated as shown at (b). If the read pixels are defaulted, the reproduced image is also irregulated as shown at (c). In the split read system, sub-scanning is performed by changing relative positional relation between the original 1 and the line sensors 2a and 3b, and hence, when originals as shown in FIG. 5 are reproduced, overlapping and defaulted portion are arrayed along straight lines l.sub.1 and l.sub.2 extending in the sub-scanning direction as shown in FIG. 6. It is to be noted that these portions are discontinuously illustrated in FIG. 6 for easy understanding of such disadvantage. Thus, such irregularity of the image is easily visualized in the above described system.
In order to cope with the disadvantage, Japanese Patent Laying-Open Gazette No. 9168/1982 proposes a technical idea of composing outputs from the line sensors 3a and 3b in a proportional distribution manner for reproducing the overlapping portions .DELTA.R. In Japanese Patent Laying-Open Gazette No. 9167/1982, further, switching positions of the two line sensors 3a and 3b are arranged at random in the sub-scanning direction on the basis of random numbers, thereby to prevent linear irregularity of the image. However, both of such processes are performed with no regard to the contents of original images, and hence switching cannot be performed in correspondence to characteristics of the originals. When, particularly in the former case, border lines etc. included in the originals are present in the overlapping portions .DELTA.R, such border lines are inevitably blurred.